The invention concerns a material for prostheses and more particularly a ceramic material for the production of prostheses.
Metallic and ceramic materials can be used for the production of prostheses, whether they are endoprostheses or exoprostheses. Chromium-cobalt steels and titanium alloys have gained entry into the prosthesis art as metallic materials. Both these materials are essentially biocompatible, being therefore compatible with the body so as to minimize rejection problems. Chromium-cobalt steels are used as a material for endoprostheses as such steels have proven themselves to be resistant to body fluids with a hydrogen ion concentration of constantly near 7 (pH 7 =neutral). On the other hand that resistance is not enjoyed in the mouth region with considerable fluctuations in pH-value. In contrast to chromium-cobalt steels titanium alloys (TiAl6N, TiAl6Nb) are resistant to fluctuations in pH-value. They are therefore used as materials for endoprostheses and for prostheses in the mouth region (exoprostheses), but a disadvantage thereof, in comparison with chromium-cobalt steels, is that those titanium alloys involve a degree of strength which is not entirely adequate. However both these metal materials can be comparatively easily mechanically worked, that is to say machined and cut.
As an alternative to such metal materials, ceramic materials have also shown themselves to be suitable for endoprostheses and exoprostheses. A number of requirements have to be made in respect of ceramic materials, for example as follows. They must be bioinert, that is to say, resistant to body fluids. To avoid the absorption of body fluids, ceramic prostheses and in particular endoprotheses should not be porous. They must also exhibit resistance to corrosion, that is to say, surface attack on or erosion of the prostheses is to be prevented. Further requirements involved are a degree of strength which is appropriate for the purpose of use of the prosthesis and biocompatibility. Ceramic materials have to comply with all the foregoing requirements, and the failure to fulfil one requirement means that a ceramic material can no longer be considered for prosthetic purposes.
Two ceramic materials have proven themselves suitable for load-bearing endoprostheses and exoprostheses, namely aluminium oxide (Al2O3) with an Al2O3 proportion of 99.85%, with the balance being other constituents, and zirconium oxide (ZrO2) of predominantly tetragonal structure, stabilized by magnesium oxide (MgO2) or by an oxide of the rare earths, preferably yttrium oxide (Y2O3) or cerium oxide (CeO2).
Clear limits are set on the use of the above-indicated ceramic materials in relation to prostheses, insofar as they appear to be less suited to the production of prostheses of complicated three-dimensional configurations, for example tooth, finger or spine part prostheses and so forth. Presintered Al2O3 semi-finished prosthesis articles which have to be subjected to dense sintering after finishing machining are distinguished by a high level of hardness and major difficulty in terms of machineability, which meant that Al2O3 was not used in the dental area, for example insofar as prostheses such as crowns and bridges may be of wall thicknesses down to a minimum of 0.2 mm. The situation is comparable with ZrO2. In sintered form it is even more difficult to machine in comparison with Al2O3. In porous form it is comparatively easy to machine and would therefore be suitable in that condition for shaping by machine of complicated three-dimensional structures, that is to say bodies, if dense sintering were not generally required in connection with prostheses, to avoid porosity. In the dental area, besides dense or consolidating sintering for eliminating porosity, such sintering is additionally indispensable for the production of prostheses involving filigree wall portions and/or thin web portions.
It is the dense or consolidating sintering of prefabricated prosthesis parts, necessitating finishing machining and cleaning, that has also imposed limits on ZrO2 in terms of its utility for the production of prostheses, insofar as it is possible to use that ceramic material for the production of relatively large load-bearing prostheses, but not small, structurally complicated prostheses such as tooth or finger prostheses. Therefore zirconium oxide (ZrO2) of the above-described kind involves a limited range of uses.